Copolymers of ethylene and an alpha-olefin comonomer produced by polymerization processes using coordination catalysts have become very successful over the last two decades because of their excellent physical, chemical, and processing properties. These copolymers have linear backbones with short chain branches resulting from the comonomer incorporated in the polymer backbone. Using certain types of catalysts and processing conditions linear backbone copolymers may be produced which in addition contain long chain branches. For the purposes of the present application all of these copolymers will be referred to as linear low density polyethylenes (LLDPE). LLDPE's have been used for a wide variety of applications using different melt processing techniques, such as blow molding, cast molding, injection molding, rotational molding, thermoforming, to produce articles of manufacture such as films, bottles, pipe, tubing, and wire and cable coating applications and derived products. Because of their excellent physical properties and processing characteristics LLDPE films are well suited for a broad spectrum of applications. Fabrication of LLDPE film is generally effected by blown film or slot casting processes. The resulting film is characterized by good physical properties such as tensile strength, high ultimate elongation, and impact strength. In particular, ethylene copolymers containing the higher alpha-olefin comonomer 1-octene are widely recognized for their excellent physical properties.
LLDPE films are useful in the packaging of frozen foods and in display packaging and require for these applications polymer compositions that are capable of shielding often irregularly shaped objects from the environment, without tearing or rupturing, and in general while maintaining good optics. An improvement of mechanical properties per unit film thickness would allow the reduction of the thickness of the films used, and thus the costs of the packaging materials. The resistance to tearing or rupturing could be improved by lowering the density of the LLDPE, but this leads to a shift in the balance between several mechanical properties, such as puncture resistance and tensile strength. Furthermore, more comonomer is required to lower the density, which would result in higher production costs. In addition, lower density LLDPE's generally have higher amounts of soluble materials which can come into contact with the articles to be packaged, such as foods. EP 1 231 228 A1 (Joubert et al) describes a process for producing co- and terpolymers of ethylene and linear alpha olefins having more than 5 carbon atoms wherein ethylene or the linear alpha olefin component is Fischer-Tropsch derived so that it includes other olefinic components. According to EP 1 231 228 A1, such polymers have a wide range of fundamental properties which are attributed by the inventors to the small amounts of other olefins present in the Fischer-Tropsch derived linear alpha olefin component which previously had been considered to be impurities. In Example 12 of EP 1 231 228 A1 a process for producing an ethylene/1-heptene copolymer is disclosed, in which the 1-heptene is derived by a Fischer-Tropsch process.
There is a continued need for materials suitable for making films that are capable of packaging or protecting objects, which are often irregularly shaped, from the environment, without tearing or rupturing, and in general while maintaining good optical properties.